Process for dehydrating gas

ABSTRACT

A process for the removal of water from gas which comprises an absorption step of bringing a gas saturated with water vapor into gas-liquid contact with a water-lean absorbing liquid comprising a water absorbing liquid having a cloud point temperature above the freezing point of water whereby water vapor present in the gas is absorbed into the water-lean absorbing liquid at a temperature below its cloud point to produce a refined gas having a reduced water vapor content and water-loaded absorbing liquid. A regeneration step is provided in which the water-loaded absorbing liquid is heated to above the cloud point temperature of the absorbing liquid whereby the water-loaded absorbing liquid separates into a water-rich phase and an absorbing liquid-rich phase and the absorbing liquid-rich phase is cooled to a temperature below its cloud point prior to recycling the absorbing liquid-rich phase for use as water-lean absorbing liquid in the absorption step.

This application is the U.S. national phase of international applicationPCT/GB02/00999, filed 6 Mar. 2002, which designated the U.S.

BACKGROUND OF THE INVENTION

This invention relates to a process for dehydrating gas, in particular,natural gas.

Conventional processes for the dehydration of natural gas involveabsorption of water into a solvent, such as triethylene glycol.Regeneration of the solvent is achieved by distilling off the absorbedwater which requires large amounts of energy. Also, an undesirable sideeffect of such processes is co-vaporization of aromatics (absorbed fromthe natural gas) with the water in the distillation step. It istherefore necessary to recover the aromatic compounds before dischargingthe water to the environment.

SUMMARY OF THE INVENTION

The present invention relates to a process for the removal of water froma gas which comprises:

-   -   (a) an absorption step of bringing a gas saturated with water        vapour into gas-liquid contact with a water-lean absorbing        liquid comprising a water absorbing liquid having a cloud point        temperature above the freezing point of water whereby water        vapour present in the gas is absorbed into the water-lean        absorbing liquid at a temperature below its cloud point to        produce a refined gas having a reduced water vapour content and        water-loaded absorbing liquid; and    -   (b) a regeneration step of heating the water-loaded absorbing        liquid to above the cloud point temperature of the absorbing        liquid whereby the water-loaded absorbing liquid separates into        a water-rich phase and an absorbing liquid-rich phase and        cooling the absorbing liquid-rich phase to a temperature below        its cloud point prior to recycling the absorbing liquid-rich        phase for use as water-lean absorbing liquid in the absorption        step.

An advantage of the process of the present invention is that water maybe separated from the water-loaded absorbing liquid to form a water-richphase at a temperature below that required to distill water from theabsorbing liquid thereby resulting in a reduced energy consumptioncompared with a conventional dehydration process. A further advantageassociated with regenerating the absorbing liquid at a relatively lowtemperature is that degradation of the absorbing liquid may be reducedor even eliminated.

It is envisaged that the gas which is saturated with water vapour mayhave condensed water entrained therein and that this entrained condensedwater may be absorbed into the water-lean absorbing liquid together withthe water vapour.

The gas saturated with water vapour may have a water content (watervapour and optionally entrained condensed water) of 10 kg per m³.

Preferably, the refined gas has a water content of less than 2 kg perm³, more preferably less than 1 kg per m³, most preferably, less than0.15 kg per m³.

Preferably, the gas which is saturated with water vapour is at apressure of at least 5 bar absolute, more preferably at least 10 barabsolute, most preferably at least 20 bar absolute, for example, atleast 30 bar absolute.

A further advantage of the process of the present invention is thatwhere the gas which is saturated with water vapour is at high pressure,the regeneration step can be achieved without a reduction in pressure ofthe water-loaded absorbing liquid or with a partial reduction in itspressure. Accordingly, the absorbing liquid-rich phase may be recycledto the absorption step (for use as water-lean absorbing liquid) at arelatively high pressure (thereby reducing or even eliminated therequirement for pressurising the water-lean absorbing liquid).

Preferably, the gas saturated with water vapour is natural gas or may beair from an air conditioning system or gas from a gas conditioningsystem or gas from a gas liquefaction system.

Where the gas saturated with water vapour is natural gas, it isenvisaged that small amounts of hydrocarbons (gaseous hydrocarbons,vaporised hydrocarbons or entrained condensed hydrocarbons, for example,vaporised aromatic compounds or entrained condensed aromatic compounds)may be absorbed into the absorbing liquid together with the water.Preferably, the water-loaded absorbing liquid has a hydrocarbon contentof less than 5% by weight, preferably less than 2.5% by weight, mostpreferably less than 1% by weight. An advantage of using the process ofthe present invention to remove water from natural gas is that anyaromatic compounds which are absorbed into the water loaded absorbingliquid will partition into the absorbing liquid-rich phase during theregeneration step. There is no requirement to separate these aromaticcompounds from the absorbing liquid-rich phase prior to recycle of theabsorbing liquid rich phase to the absorption step. Accordingly, theprocess of the present invention results in a reduction in the emissionof these harmful chemicals into the environment and/or eliminates theneed for an aromatics recovery step.

The cloud point temperature of the water absorbing liquid is defined asthe temperature at which the water-loaded absorbing liquid no longerexhibits single phase behaviour and becomes cloudy as the water startsto separate from the absorbing liquid. The cloud point temperature istherefore an indication of the temperature at which the water-loadedabsorbing liquid will be expected to separate into a water-rich phaseand an absorbing liquid-rich phase. The cloud point temperature isdependent upon the nature of the absorbing liquid and its water loadingand is substantially insensitive to the pressure of the regenerationstep. It will be appreciated that the absorbing liquid should beselected to suit the temperature of the gas from which the water is tobe removed. Thus, the gas which is saturated with water vapour may be atan elevated temperature, ambient temperature, or may be chilled to atemperature below ambient conditions.

Suitably, the cloud point temperature of the absorbing liquid is in therange 1 to 120° C., preferably 10 to 90° C., more preferably 15 to 80°C., for example, 20 to 40° C.

Preferably, the absorption step takes place at a temperature which is atleast 5° C. below the cloud point temperature of the absorbing liquid,more preferably, at least 10° C., most preferably at least 15° C. belowthe cloud point temperature. Where the gas which is brought into contactwith the absorption liquid is at a temperature below the freezing pointof water, it is essential that the absorbing liquid acts as ananti-freeze for water at the temperature of the absorption step. It isalso essential that the cloud point temperature of the absorbing liquid(and hence the temperature of the regeneration step) is above thefreezing point of water so as to avoid freezing of the water as itseparates from the absorbing fluid.

In order to achieve a good separation of the water-rich phase and theabsorbing liquid-rich phase, it is preferred to carry out theregeneration step at a temperature which is at least 2.5° C. above thecloud point temperature of the absorbing liquid, more preferably, atleast 5° C. above the cloud point temperature.

Suitably, the regeneration step is carried out in a separator, forexample a settling unit, decanter, hydrocyclone, electrostatic coalesceror centrifuge.

The water loaded absorbing liquid which is fed to the separator may beheated prior to entering the separator and/or is heated in theseparator. It is preferred that the temperature of the water loadedabsorbing liquid is maintained below its cloud point temperature untilit enters the separator in order to mitigate the risk of premature phaseseparation leading to flow (e.g. slugging flow) and corrosion problems.

Where the gas which is saturated with water vapour contains carbondioxide, it is envisaged that the absorbing liquid may also be selectivefor the absorption of carbon dioxide. The absorbed carbon dioxide may beseparated from the water-loaded absorbing liquid by heating thewater-loaded absorbing liquid. For example, the water-loaded absorbingliquid may be fed to a heated separator having a headspace therein suchthat absorbed carbon dioxide is liberated into the headspace under theaction of heating. Optionally, the regeneration step is carried out witha reduction in the pressure of the water-loaded absorbing fluid in orderto facilitate the liberation of carbon dioxide from the absorbingliquid. Suitably, the carbon dioxide is liberated from the water-loadedabsorbing liquid at a temperature in the range 50 to 100° C., preferably60 to 80° C. Suitably, the carbon dioxide is liberated from thewater-loaded absorbing liquid at a pressure in the range 5 to 100 barabsolute, preferably 10 to 70 bar absolute. Where the regeneration stepis carried out without a reduction in pressure of the water-loadedabsorbing liquid (or with a partial reduction in its pressure), theseparated absorbing liquid-rich phase may be subjected to a reduction inpressure, optionally with heating, in order to liberate any carbondioxide which remains absorbed therein. Suitably, the separatedabsorbing-liquid rich phase is depressurized to a pressure in the range1 to 30 bar absolute, preferably 5 to 20 bar absolute. Suitably, theseparated absorbing-liquid rich phase is heated to a temperature in therange 50 to 100° C., preferably 60 to 80° C.

Suitably, the water absorbing liquid may be an amine, ether, alcohol,ester, carbonate or mixtures thereof.

Preferred amines include hexylamine, octylamine, nonylamine, N-methyl,diethylamine, N-methyl isopropylamine, N-methyl diisopropylamine,dipropylamine, diisopropylamine, di(prop-2-ene)amine, N-methyln-butylamine, N-methyl n-pentylamine, N-methyloctylamine, amines ofgeneral formula NHR¹R² (wherein R¹ is ethyl and R² may be selected fromethyl, n-propyl, isopropyl, n-butyl, iso-butyl, and tert-butyl),N,N-dimethylethylamine, N,N-dimethylpropylamine,N,N-dimethylisopropylamine, N,N-dimethyl(prop-2-ene)amine,N,N-dimethylisobutylamine, N,N-dimethyltertbutylamine,N,N-diethylmethylamine, ethylmethylisopropylamine, triethylamine, andN-allyl-dimethylamine. Particularly preferred amines are triethylamineand N-allyl-dimethylamine. Other suitable amines include diamines ofgeneral formula R³ ₂(CH₂)_(n)NR³ ₂ wherein R³ is methyl or ethyl and nis an integer in the range 2 to 6. Preferred diamines includeN,N,N′,N′-tetraethyl-1,2-ethanediamine,N,N,N′,N′-tetraethyl-1,2-ethanediamine,N,N,N′N′-tetramethyl-1,6-heanediamine andN,N,N′,N′-tetraethyl-1,3-propanediamine. The amine may also be analkanolamine such as monoethanolamine (2-hydroxyethylamine),diethanolamine (bis(2-hydroxyethyl)amine)triethanolamine(tris(2-hydroxyethyl)amine), N-methyldiethylethanolamine, diglycolamine,and alkanolamines of the general formula R⁴ ₂NCH(R⁵)CH(R⁵)OH (whereineach R⁴ is independently selected from the group consisting of ethyl,n-propyl, isopropyl, n-butyl, isobutyl and t-butyl and R⁵ is H ormethyl). Preferred alkanolamines include 2-(diisopropylamino)ethanol,and 1-diethylamino-2-propanol.

Preferred ethers are of general formula R⁶O(CH₂CH₂O)_(n)R⁷ wherein R⁶ isselected from the group consisting of H, methyl and ethyl, R⁷ isselected from the group consisting of ethyl, n-propyl, isopropyl,n-butyl, isobutyl and t-butyl and n is 1 or 2. Particularly preferredethers of this general formula include ethylene glycol butyl ether(2-butoxyethanol), diethylene glycol diethyl ether (2-ethoxyethylether), and diethylene glycol tert-butyl methyl ether. Other suitableethers are glycerol ethers of general formula R⁸OCH₂CH(OH)CH₂OR⁹(wherein R⁸ is methyl or ethyl and R⁹ is selected from n-propyl,isopropyl, n-butyl, isobutyl and t-butyl) such as ethyl n-propylglycerol ether, ethyl isopropyl glycerol ether, methyl n-butyl glycerolether, ethyl n-butyl glycerol ether, methyl tert-butyl glycerol ether,diisopropyl glycerol ether, methyl n-amyl glycerol ether, methyl isoamylglycerol ether, methyl 2-amyl glycerol ether, propylene glycol n-propylether, and ethyl n-hexyl oxyethyl glycerol ether.

Preferred alcohols include 2-butanol, cyclohexanol, cyclohexylmethanol,benzyl alcohol, 3,3-dimethyl-2-butanol, 3-heptanol, 2-octanol,3-octanol, 1-nonanol, 2-nonanol, 1-decanol, 2-methylcyclohexanol,2-ethyl-1-butanol, 3,3,5-trimethyl-1-hexanol, 3,5,5-trimethyl-1-hexanol,2,6-dimethyl-4-heptanol, 3-methyl-1-pentanol, 5-methyl-2-hexanol,cyclopentanol, 3-methylcyclohexanol, 4-methylcyclohexanol,2,6-dimethylcyclohexanol, cycloheptanol, cyclooctanol,2-methyl-1-propanol, 1-butanol, 4-heptanol, 1-undecanol, 1-dodecanol,1-phenyl-1-propanol, and diols such as 1,3-dimethylbutoxypropanediol.

Preferred esters include esters propyl formate, isopropyl formate, butylformate, isobutyl formate, pentyl formate, cyclohexyl formate, isoamylformate, hexyl formate, heptyl formate, octyl formate, ethyl acetate,propyl acetate, isopropyl acetate, sec-butyl acetate, isobutyl acetate,butyl acetate, tert-butyl acetate, pentyl acetate, isopentyl acetate,hexyl acetate, cyclohexyl acetate, heptyl acetate, isononyl acetate,benzyl acetate, methyl chloroacetate, ethyl chloroacetate, methyldichloroacetate, methyl trichloroacetate, methyl trimethylacetate, ethyltrimethylacetate, methyl propionate, ethyl propionate, propylpropionate, methyl 2-chloropropionate, butyl propionate, isopentylpropionate, hexyl propionate, cyclohexyl propionate, methyl butyrate,methyl isobutyrate, ethyl isobutyrate, ethyl butyrate, propyl butyrate,isopropyl butyrate, butyl butyrate, butyl isobutyrate, isobutylisobutyrate, methyl 4-chlorobutyrate, pentyl butyrate, isopentylbutyrate, hexyl isobutyrate, methyl enanthate, methyl caproate, ethylcaproate, ethyl caprylate, methyl valerate, ethyl isovalerate, diethyloxalate, diethyl succinate, dimethyl glutarate, dimethyl adipate,diethyl adipate, methyl salicylate, ethyl salicylate, dimethyl maleate,methyl benzoate, ethyl benzoate and esters of the general formulaR¹⁰C(O)O(CH₂CH₂O)_(n)R¹¹ (wherein R¹⁰ is methyl or ethyl, R¹¹ isselected from the group consisting of ethyl, n-propyl, isopropyl,n-butyl, isobutyl and t-butyl and n is 1 or 2) such as diethylene glycolethyl ether acetate.

Preferred carbonates include dimethyl carbonate and diethyl carbonate.

The water absorbing liquid may also be an ethylene oxide homopolymer ora copolymer of ethylene oxide and propylene oxide having a molecularweight in the range 10,000 to 100,000. The polymers may be used as suchor may be dissolved in a protic solvent. Suitable protic solventsinclude alcohols (for example, methanol and ethanol) glycols (forexample, ethylene glycol monobutyl ether), dimethylformamide, anddimethylsulfoxide.

The ethylene oxide homopolymer may have the following general formula:R¹—(CH₂CH₂O)_(m)—R²wherein R¹ may be selected from H, or a C₁ to C₆ straight chain orbranched chain aliphatic group, R² is a C₁ to C₆ straight chain orbranched chain aliphatic group, and m is an integer. Where R¹ is a C₁ toC₆ straight or branched chain aliphatic groups, R¹ and R² may be thesame or different. Preferred C₁ to C₆ straight or branched chainaliphatic groups include methyl, ethyl, n-propyl, isopropyl, n-butyl,isobutyl and tert-butyl.

Alternatively, the ethylene oxide homopolymer may have the generalformula:R³—(CH₂CH₂O)_(y)—R⁴wherein R³ is a C₂ to C₁₆ straight chain or branched chain aliphaticgroup, R⁴ is H or a C₁ to C₁₆ straight chain or branched chain aliphaticgroup and y is an integer in the range 2 to 16.

The copolymer of ethylene oxide and propylene oxide may be a randomblock copolymer of the following general formula:R¹—(CH₂CH₂O)_(m)(CH₂CHMeO)_(n)—R²wherein R¹ and R² are as defined above, Me is methyl, and m and n areintegers.

Preferably, the water-loaded absorbing liquid, which is heated in theregeneration step, has a water loading of at least 5% by weight, morepreferably at least 7.5% by weight, most preferably, at least 10% byweight. For example, the water-loaded absorbing liquid may have a waterloading in the range 10 to 30% by weight, preferably 10 to 20% byweight.

The absorbing liquid-rich phase may contain low amounts of water.Preferably, the absorbing liquid-rich phase contains less than 2.5%weight of water, more preferably less than 1% weight, most preferablyless than 0.5% weight of water. Where the water-lean absorbing liquidcontains such low amounts of water, there is no requirement to removethe water contaminant prior to recycling the absorbing liquid to theabsorption step. However, where the absorbing liquid-rich phase containshigher amounts of water (for example, greater than 5% weight of water)it may be necessary to distill at least a portion of the water from theabsorbing liquid prior to recycling the absorbing liquid to theabsorption step. Nevertheless, the energy consumed in distilling thewater contaminant from the absorbing liquid-rich phase is less than thatwhich would be required to remove water from the water-loaded absorbingliquid solely by the action of distillation.

The water-rich phase may contain low amounts of absorbing liquid.Preferably, the water-rich phase contains less than 2.5% weight ofabsorbing liquid, more preferably, less than 1% weight and mostpreferably less than 0.5% weight of absorbing liquid. Preferably, atleast a portion of the contaminating absorbing liquid, preferablysubstantially all of the contaminating absorbing liquid is removed fromthe water-rich phase in a water purification step. The absorbing liquidwhich is removed from the water-rich phase is then recycled to theabsorption step.

Suitably, the water purification step comprises contacting thewater-rich phase with a wash solvent which is immiscible with water suchthat at least part of the absorbing liquid present in the water-richphase is extracted into the wash solvent to produce an extract phase anda purified water phase. The extract phase is then separated from thepurified water phase. In order to facilitate separation of the washsolvent and the absorbing liquid, it is preferred that the wash solventhas a boiling point which is substantially lower than that of theabsorbing liquid thereby allowing the wash solvent to be separated fromthe absorbing liquid as an overhead fraction by distillation. Theseparated wash solvent (overhead fraction) may then recycled to thepurification step while the absorbing liquid (bottom fraction) may berecycled to the absorption step.

Where the gas which is saturated with water is natural gas, theabsorbing liquid-rich phase may contain absorbed hydrocarbons. Suitably,the extract phase from the water purification step may be combined withthe absorbing liquid-rich phase and the combined phases may be fed to adistillation column wherein the wash solvent together with any lighthydrocarbons are separated from the absorbing liquid as an overheadfraction. The overhead fraction is then cooled to condense the washsolvent and the condensed wash solvent is separated from any gaseoushydrocarbons using a conventional gas-liquid separator. The wash solventis then recycled to the water purification step. Preferably, the gaseoushydrocarbons are combined with the refined natural gas.

Suitably, the wash solvent has a boiling point which is at least 20° C.below, more preferably at least 40° C. below the boiling point of theabsorbing liquid (in order to mitigate the risk of forming an azeotropicmixture in the distillation column).

Preferred wash solvents include light end hydrocarbons having from 5 to10 carbons, preferably, from 5 to 8 carbons, and mixtures thereof.Examples of light end hydrocarbons include alkanes and cycloalkanes suchas n-pentane, methylcyclopentane, n-hexane, cyclohexane, mixtures ofhexane isomers, n-heptane, mixtures of heptane isomers, n-octane andmixtures of octane isomers. The wash solvent may also be an aromatichydrocarbon such as toluene and xylene.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an embodiment of the process ofthe present invention in which water is absorbed from a gas saturatedwith water vapour into a water lean absorbing liquid in an absorptiontower (2) and the resulting water-loaded absorbing liquid (5) isregenerated in a heated separator (6) by heating the water-loadedabsorbing liquid to above its cloud point temperature.

FIG. 2 is a modification of the process scheme illustrated in FIG. 1 inwhich a portion of the water-loaded absorption liquid which is withdrawnfrom the absorption tower (2) is combined with the regeneratedabsorption liquid (3) and is reintroduced to the absorption tower (2).

FIG. 3 is a schematic illustration of a typical counter-current flowprocess of the present invention.

The invention will now be illustrated with the aid of FIGS. 1 to 3 whichrepresent schematic diagrams of three embodiments of the process of thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1, a gas which is saturated with water vapour (1) is fed to anabsorption tower (2) at or near the bottom thereof. The absorption towermay be packed, for example, with an irregular packing material and/ormay have plates therein so that the ascending gas comes into efficientgas liquid contact with a water-lean absorbing liquid (3) fed to theupper part of the absorption tower (2). Refined gas (4) having a reducedwater vapour content, as a result of being contacted with the water-leanabsorbing liquid (3), is discharged from the top of the absorption tower(2). Water-loaded absorbing liquid (5) is withdrawn from near the bottomof the absorption tower (2) and is transferred to the regeneration stepwhere it is regenerated. The water-loaded absorbing liquid (5) may haveabsorbed therein components of the gas (1) which were absorbed togetherwith the water (such as trace amounts of hydrocarbons).

The regeneration step comprises a heated separator (6) in which thewater-loaded absorbing liquid (5) is heated to a temperature above thecloud point temperature of the absorbing liquid and separates into awater-rich phase (7) and an absorbing liquid-rich phase (8). Generally,the water-rich phase (7) will be denser than the absorbing liquid-richphase (8) in which case the water-rich phase (7) forms the bottom phasein the separator (6).

The water-rich phase (7) is withdrawn from the separator (6) and is fedto a wash column (9) near the top thereof where it is brought intoliquid-liquid contact with a wash solvent (10) having a density lowerthan that of water. The wash solvent (10) is fed to the lower part ofthe wash column (9) above a sump region (11). The water-rich phase (7)and the wash solvent (10) pass in a counter current fashion through thewash column such that the absorbing liquid which contaminates thewater-rich phase (7) is extracted into the wash solvent (10) andpurified water collects in the sump region (11). A purified water stream(12) is withdrawn from the sump region (11) while an extract stream (13)comprising wash solvent and extracted absorbing liquid is removed fromthe top of the wash column (9). Alternatively, it is envisaged that thewater-rich phase and wash solvent may be contacted in a mixing vesseland the resulting mixture may then be passed to a separator where anextract phase is separated from a purified water phase.

The extract stream (13) from the wash column (9) is combined with theabsorbing liquid-rich phase (8) which is withdrawn from at or near thetop of the heated separator (6) and the combined stream (14) is passedto a distillation column (15) where the wash solvent and any low boilingcomponents of the treated gas (which were absorbed into the absorbingliquid together with the water) are separated as an overhead fraction(16) from the absorbing liquid. The overhead fraction (16) is cooled tocondense out the wash solvent (10) which is separated from any gaseouscomponents (17) of the overhead fraction (16) in a separator (18). Thegaseous components are combined with the refined gas (4) while thecondensed wash solvent (10) is recycled to the wash column (9). Thebottom fraction from the distillation column is recycled to theabsorption tower (2) for use as water-lean absorbing liquid (3).

FIG. 2 is a modification of the process illustrated in FIG. 1 in which aportion of the water-loaded absorption liquid which is withdrawn fromthe absorption tower is combined with the regenerated absorption liquid(3) and is reintroduced into the absorption tower (2). The remainder ofthe water-loaded absorption liquid is passed to the regeneration step(not shown). This modification of the process of FIG. 1 results in anincrease in the water loading of the absorbing liquid. It is alsoenvisaged that a portion of the refined gas (4) which is withdrawn fromthe top of the absorption tower could be combined with fresh gas whichis saturated with water and that the combined gas stream could be fed tothe bottom of the absorption tower thereby further reducing the watercontent of the refined gas (not shown).

A typical continuous counter-current flow process is illustrated in FIG.3. A gas saturated with water vapour (20) is fed to a first absorptiontower (21) and a gas having a reduced water content (22) is withdrawnfrom the top of the first absorption tower (21) and is fed to the bottomof a second absorption tower (23). Regenerated water-lean absorbingliquid (24) is fed to the second absorption tower (23) while gas whichis further reduced in water content (25) is withdrawn from the top ofthe second absorption tower (23). Absorbing liquid which is partiallyloaded with water (26) is withdrawn from the second absorption tower andis fed to the first absorption tower (21). Absorbing liquid having anincreased loading of water (27) is removed from the first absorptiontower and is passed to the regeneration step (not shown). The number ofabsorption towers employed in a continuous counter-current flowconfiguration process is dependent on the level of water in the gas andthe desired water-loading of the absorbing liquid which is to be fed tothe regeneration step. Suitably, 2–6, preferably 2–3 absorption towersare employed.

EXAMPLES Example 1

The results given below in Table 1 show the effect of water loading onthe cloud point temperature of a mixture of water andN-allyl-dimethylamine.

TABLE 1 % amine (by % water (by Cloud Point Amine (g) Water (g) weight)weight) Temperature (° C.) 1 9 10 90 57.9 2 9 18.2 81.8 48 3 9 25 7547.5 5 9 35.7 64.3 47.6 7 9 43.8 56.2 47.8 10 9 52.6 47.4 49.1 6.39 0.592.7 7.3 — 6.39 1 86.5 13.5 — 6.39 2 76.2 23.8 59.2 6.39 4 61.5 38.550.4 6.39 5 56.1 43.9 49.1

It can be seen that at water loadings of between 38.5 and 81.8% weight,the cloud point temperature remains relatively constant at approximately50° C. and that the cloud point temperature increases dramatically atwater loadings outside of this range. Accordingly, N-allyl-dimethylamineis suitable for use as a water absorbing fluid in the process of thepresent invention.

Example 2

The results given below in Table 2 show the effect of water loading onthe cloud point temperature of a mixture of water and triethylamine.

TABLE 2 Cloud Point % amine (by % water (by Temperature Amine (g) Water(g) weight) weight) (° C.) 9.9 0.1 99 1 — 9.8 0.2 98 2 65 9.6 0.4 96 438.7 9.5 0.5 95 5 20 9 1 90 10 18.5 8 2 80 20 17.4 7 3 70 30 17.8 6 4 6040 17.7 5 5 50 50 18 4 6 40 60 17.2 3 7 30 70 17.4 2 8 20 80 17.2 1 9 1090 19.6 0.5 9.5 5 95 32.8 0.4 9.6 4 96 39.2 0.2 9.8 2 98 — 0.1 9.9 1 99—

It can be seen that at water loadings of between 5 and 90% by weight,the cloud point temperature remains relatively constant at between17–20° C. and that the cloud point temperature increases dramatically atwater loadings outside of this range. Accordingly, triethylamine ishighly suitable for use as a water absorbing fluid in the process of thepresent invention.

Example 3

The results given below in Table 3 show the effect of water loading onthe cloud point temperature of a mixture of water andN,N,N′,N′-tetraethyl-1,2-ethanediamine.

TABLE 3 Cloud Point % amine (by % water (by Temperature Amine (g) Water(g) weight) weight) (° C.) 9.5 0.5 95 5 — 9 1 90 10 14.1 8 2 80 20 25.47 3 70 30 25.8 6 4 60 40 26.4 5 5 50 50 26.9 4 6 40 60 28.2 3 7 30 7028.8 2 8 20 80 31.8 1 9 10 90 38.9 0.5 9.5 5 95 48.8

It can be seen that at water loadings of between 20% and 70% weight, thecloud point temperature remains relatively constant at approximately 25to 29° C. and that the cloud point temperature increases dramatically atwater loadings above 70% weight. Accordingly,N,N,N′,N′-tetraethyl-1,2-ethanediamine is suitable for use as a waterabsorbing fluid in the process of the present invention.

Example 4

The results given below in Table 4 show the effect of water loading onthe cloud point temperature of a mixture of water andN,N,N′,N′-tetramethyl-1,6-hexanediamine

TABLE 4 Cloud Point % amine (by % water (by Temperature Amine (g) Water(g) weight) weight) (° C.) 9.5 0.5 95 5 100 9 1 90 10 100 8 2 80 20 74.57 3 70 30 67.3 6 4 60 40 62.1 5 5 50 50 58.9 4 6 40 60 57.2 3 7 30 7056.4 2 8 20 80 56.4 1 9 10 90 56.5 0.5 9.5 5 95 60.3

It can be seen that at water loadings of between 30% and 95% weight, thecloud point temperature remains relatively constant at approximately 60°C. and that the cloud point temperature increases dramatically at waterloadings below 30% weight. Accordingly,N,N,N′,N′-tetramethyl-1,6-hexanediamine is suitable for use as a waterabsorbing fluid in the process of the present invention.

Example 5

The results given below in Table 5 show the effect of water loading onthe cloud point temperature of a mixture of water andN,N,N′,N′-tetraethyl-1,3-propanediamine.

TABLE 5 Cloud Point % amine (by % water (by Temperature Amine (g) Water(g) weight) weight) (° C.) 9.5 0.5 95 5 12.5 9 1 90 10 18.4 8 2 80 2021.0 7 3 70 30 21.6 6 4 60 40 21.6 5 5 50 50 21.6 4 6 40 60 21.8 3 7 3070 21.9 2 8 20 80 21.7 1 9 10 90 23.9 0.5 9.5 5 95 32.7

It can be seen that at water loadings of between 10% and 90% weight, thecloud point temperature remains relatively constant at approximately 22°C. and that the cloud point temperature increases at water loadingsabove 90% weight. Accordingly, N,N,N′,N′-tetraethyl-1,3-propanediamineis suitable for use as a water absorbing fluid in the process of thepresent invention.

Example 6

The results given below in Table 6 show the effect of water loading onthe cloud point temperature of a mixture of water and2-(diisopropylamino)ethanol.

TABLE 6 % Cloud Point Alkanolamine alkanolamine % water (by Temperature(g) Water (g) (by weight) weight) (° C.) 9.5 0.5 95 5 100 9 1 90 10 29.18 2 80 20 17.2 7 3 70 30 14.5 6 4 60 40 13.6 5 5 50 50 13.6 4 6 40 6013.6 3 7 30 70 13.6 2 8 20 80 15.6 1 9 10 90 21.9 0.5 9.5 5 95 41.4

It can be seen that at water loadings of between 30 and 80% by weight,the cloud point temperature remains relatively constant at between13.6–15.6° C. and that the cloud point temperature increases at waterloadings outside of this range. Accordingly, 2-(diisopropylamino)ethanolis highly suitable for use as a water absorbing fluid in the process ofthe present invention.

Example 7

The results given below in Table 7 show the effect of water loading onthe cloud point temperature of a mixture of water and1-diethylamino-2-propanol.

TABLE 7 % Cloud Point Alkanolamine alkanolamine % water (by Temperature(g) Water (g) (by weight) weight) (° C.) 9.5 0.5 95 5 100 9 1 90 10 42.78 2 80 20 30 7 3 70 30 29.4 6 4 60 40 29.3 5 5 50 50 29.6 4 6 40 60 29.73 7 30 70 30.5 2 8 20 80 35 1 9 10 90 50.4 0.5 9.5 5 95 75.8

It can be seen that at water loadings of between 20 and 70% by weight,the cloud point temperature remains relatively constant at between29.3–30.5° C. and that the cloud point temperature increasesdramatically at water loadings outside of this range. Accordingly,1-diethylamino-2-propanol is highly suitable for use as a waterabsorbing fluid in the process of the present invention.

Example 8

The results given below in Table 8 show the effect of water loading onthe cloud point temperature of a mixture of water and ethylene glycolbutyl ether.

TABLE 8 Cloud Point Glycol ether % glycol ether % water (by Temperature(g) Water (g) (by weight) weight) (° C.) 9.5 0.5 95 5 100 9 1 90 10 1008 2 80 20 100 7 3 70 30 100 6 4 60 40 100 5 5 50 50 55.2 4 6 40 60 52.53 7 30 70 49.5 2 8 20 80 49.3 1 9 10 90 66.4 0.5 9.5 5 95 100

It can be seen that at water loadings of between 60 and 80% by weight,the cloud point temperature remains relatively constant at between49.3–52.5° C. and that the cloud point temperature increases at waterloadings outside of this range. Accordingly, ethylene glycol butyl etheris highly suitable for use as a water absorbing fluid in the process ofthe present invention.

Example 9

The results given below in Table 9 show the effect of water loading onthe cloud point temperature of a mixture of water and2-ethoxyethylether.

TABLE 9 Cloud Point % ether (by % water (by Temperature Ether (g) Water(g) weight) weight) (° C.) 9.5 0.5 95 5 100 9 1 90 10 100 8.5 1.5 85 1537.6 8 2 80 20 29.9 7 3 70 30 27.5 6 4 60 40 27.8 5 5 50 50 31.8 4 6 4060 40.9 3 7 30 70 48.9 2 8 20 80 62 1 9 10 90 90.7 0.5 9.5 5 95 100

It can be seen that at water loadings of between 20 and 50% by weight,the cloud point temperature remains relatively constant at between27.5–31.8° C. and that the cloud point temperature increases at waterloadings outside of this range. Accordingly, 2-ethoxyethylether ishighly suitable for use as a water absorbing fluid in the process of thepresent invention.

Example 10

The results given below in Table 10 show the effect of water loading onthe cloud point temperature of a mixture of water and diethylene glycoltert-butyl methyl ether.

TABLE 10 Cloud Point Glycol ether % glycol ether % water (by Temperature(g) Water (g) (by weight) weight) (° C.) 9.5 0.5 95 5 100 9 1 90 10 44.38 2 80 20 19.9 7 3 70 30 19.4 6 4 60 40 19.4 5 5 50 50 20.8 4 6 40 60 243 7 30 70 28.3 2 8 20 80 38.9 1 9 10 90 51.9 0.5 9.5 5 95 71.7

It can be seen that at water loadings of between 20 and 50% by weight,the cloud point temperature remains relatively constant at between 19.4and 20.8° C. and that the cloud point temperature increases at waterloadings outside of this range. Accordingly, diethylene glycoltert-butyl methyl ether is highly suitable for use as a water absorbingfluid in the process of the present invention.

Example 11

The results given below in Table 11 show the effect of water loading onthe cloud point temperature of a mixture of water and diethylene glycolmonoethyl ether acetate.

TABLE 11 Cloud Point Glycol ether % glycol ether % water Temperature (g)Water (g) (by weight) (by weight) (° C.) 9.5 0.5 95 5 100 9 1 90 10 1008 2 80 20 85.5 7 3 70 30 59.4 6 4 60 40 47.8 5 5 50 50 41.7 4 6 40 6040.2 3 7 30 70 40.8 2 8 20 80 52.6 1 9 10 90 100 0.5 9.5 5 95 100

It can be seen that at water loadings of between 50 and 70% by weight,the cloud point temperature remains relatively constant at between40.2–41.7° C. and that the cloud point temperature increases at waterloadings outside of this range. Accordingly, diethylene glycol monoethylether acetate is suitable for use as a water absorbing fluid in theprocess of the present invention.

1. A process for the removal of water from a gas which comprises: (a) anabsorption step of bringing a gas saturated with water vapour intogas-liquid contact with a water-lean absorbing liquid comprising a waterabsorbing liquid whereby water vapour present in the gas is absorbedinto the water-lean absorbing liquid to produce a refined gas having areduced water vapour content and a water-loaded absorbing liquid havinga cloud point temperature above the freezing point of water wherein thetemperature of the absorption step is below the cloud point temperatureof the water-loaded absorbing fluid; and (b) a regeneration step ofheating the water-loaded absorbing liquid to above the cloud pointtemperature of the water-loaded absorbing liquid whereby thewater-loaded absorbing liquid separates into a water-rich phase and anabsorbing liquid-rich phase and cooling the absorbing liquid-rich phaseto a temperature below its cloud point prior to recycling the absorbingliquid-rich phase for use as water-lean absorbing liquid in theabsorption step.
 2. A process according to claim 1 wherein the gassaturated with water vapour has a water content of 10 kg per m³.
 3. Aprocess according to claim 1 wherein the refined gas has a water contentof less than 2 kg per m³.
 4. A process according to claim 3 wherein therefined gas has a water content of less than 0.15 kg per m³.
 5. Aprocess according to claim 1 wherein the gas which is saturated withwater vapour is at a pressure of at least 20 bar absolute.
 6. A processaccording to claim 1 wherein the gas saturated with water vapour isselected from natural gas, air from an air conditioning system, gas froma gas conditioning system and gas from a gas liquefaction system.
 7. Aprocess as claimed in claim 6 wherein the gas saturated with watervapour is natural gas and the water-loaded absorbing liquid has ahydrocarbon content of less than 5% by weight.
 8. A process as claimedin claim 1 wherein the cloud point temperature of the water-loadedabsorbing liquid is in the range 1 to 120° C.
 9. A process as claimed inclaim 8 wherein the cloud point temperature of the water-loadedabsorbing liquid is in the range 15 to 80° C.
 10. A process as claimedin claim 1 wherein the absorption step takes place at a temperaturewhich is at least 5° C. below the cloud point temperature of thewater-loaded absorbing liquid.
 11. A process as claimed in claim 10wherein the absorption step takes place at a temperature which is atleast 15° C. below the cloud point temperature of the water-loadedabsorbing liquid.
 12. A process as claimed in claim 1 wherein theabsorption and regeneration steps take place at a temperature below andabove the freezing point of water respectively and the absorbing liquidis an anti-freeze for water at the temperature of the absorption step.13. A process as claimed in claim 1 wherein the regeneration step iscarried out at a temperature which is at least 2.5° C. above the cloudpoint temperature of the water-loaded absorbing liquid.
 14. A process asclaimed in claim 13 wherein the regeneration step is carried out at atemperature which is at least 5° C. above the cloud point temperature ofthe water-loaded absorbing liquid.
 15. A process as claimed in claim 1wherein the regeneration step is carried out in a separator selectedfrom a settling unit, decanter, hydrocyclone, electrostatic coalescerand a centrifuge.
 16. A process as claimed in claim 1 wherein the gaswhich is saturated with water vapour contains carbon dioxide, theabsorbing liquid is selective for the absorption of carbon dioxide andthe absorbed carbon dioxide is separated from the water-loaded absorbingliquid by heating the water-loaded absorbing liquid to a temperature inthe range 60 to 80° C. at a pressure in the range 10 to 70 bar absolute.17. A process as claimed in claim 1 wherein the water absorbing liquidis an amine, ether, alcohol, ester, carbonate, an ethylene oxidehomopolymer having a molecular weight in the range 10,000 to 100,000, acopolymer of ethylene oxide and propylene oxide having a molecularweight in the range 10,000 to 100,000 or mixtures thereof.
 18. A processas claimed in claim 17 wherein the amine is selected from hexylamine,octylamine, nonylamine, N-methyl, diethylamine, N-methyl isopropylamine,N-methyl diisopropylamine, dipropylamine, diisopropylamine,di(prop-2-ene)amine, N-methyl n-butylamine, N-methyl n-pentylamine,N-methyloctylamine, amines of general formula NHR¹R² (wherein R¹ isethyl and R² may be selected from ethyl, n-propyl, isopropyl, n-butyl,iso-butyl, and tert-butyl), N,N-dimethylethylamine,N,N-dimethylpropylamine, N,N-dimethylisopropylamine,N,N-dimethyl(prop-2-ene)amine, N,N-dimethylisobutylamine,N,N-dimethyltertbutylamine, N,N-diethylmethylamime,ethylmethylisopropylamine, triethylamine, N-allyl-dimethylamine diaminesof general formula R³ ₂(CH₂)_(n)NR³ ₂ (wherein R³ is methyl or ethyl andn is an integer in the range 2 to 6), monoethanolamine, diethanolamine,triethanolamine, N-methyldiethylethanolamine, diglycolamine, andalkanolamines of the general formula R⁴ ₂NCH(R⁵)CH(R⁵)OH (wherein eachR⁴ is independently selected from the group consisting of ethyl,n-propyl, isopropyl, n-butyl, isobutyl and t-butyl and R⁵ is H ormethyl).
 19. A process as claimed in claim 17 wherein the ether is ofgeneral formula R⁶O(CH₂CH₂O)_(n)R⁷ wherein R⁶ is selected from the groupconsisting of H, methyl and ethyl, R⁷ is selected from the groupconsisting of ethyl) n-propyl, isopropyl, n-butyl, isobutyl and t-butyland n is 1 or 2 or is a glycerol ether of general formulaR⁸OCH₂CH(OH)CH₂OR⁹ wherein R⁸ is methyl or ethyl and R⁹ is selected fromn-propyl, isopropyl, n-butyl, isobutyl and t-butyl.
 20. A process asclaimed in claim 17 wherein the alcohol is selected from the groupconsisting of 2-butanol, cyclohexanol, cyclohexylmethanol, benzylalcohol, 33-dimethyl-2-butanol, 3-heptanol, 2-octanol, 3-octanol,1-nonanol, 2-nonanol, 1-decanol, 2-methylcyclohexanol,2-ethyl-1-butanol, 3,3,5-trimethyl-1-hexanol, 3,5,5-trimethyl-1-hexanol,2,6-dimethyl-4-heptanol, 3-methyl-1-pentanol, 5-methyl-2-hexanol,cyclopentanol, 3-methylcyclohexanol, 4-methylcyclohexanol,2,6-dimethylcyclohexanol, cycloheptanol, cyclooctanol,2-methyl-1-propanol, 1-butanol, 4-heptanol, 1-undecanol, 1-dodecanol,1-phenyl-1-propanol, and diols such as 1,3-dimethylbutoxypropanediol.21. A process as claimed in claim 17 wherein the ester is selected fromthe group consisting of propyl formate, isopropyl formate, butylformate, isobutyl formate, pentyl formate, cyclohexyl formate, isoamylformate, hexyl formate, heptyl formate, octyl formate, ethyl acetate,propyl acetate, isopropyl acetate, sec-butyl acetate, isobutyl acetate,butyl acetate, tert-butyl acetate, pentyl acetate, isopentyl acetate,hexyl acetate, cyclohexyl acetate, heptyl acetate, isononyl acetate,benzyl acetate, methyl chloroacetate, ethyl chloroacetate, methyldichloroacetate, methyl trichloroacetate, methyl trimethylacetate, ethyltrimethylacetate, methyl propionate, ethyl propionate, propylpropionate, methyl 2-chloropropionate, butyl propionate, isopentylpropionate, hexyl propionate, cyclohexyl propionate, methyl butyrate,methyl isobutyrate, ethyl isobutyrate, ethyl butyrate, propyl butyrate,isopropyl butyrate, butyl butyrate, butyl isobutyrate, isobutylisobutyrate, methyl 4-chlorobutyrate, pentyl butyrate, isopentylbutyrate, hexyl isobutyrate, methyl enanthate, methyl caproate, ethylcaproate, ethyl caprylate, methyl valerate, ethyl isovalerate, diethyloxalate, diethyl succinate, dimethyl glutarate, dimethyl adipate,diethyl adipate, methyl salicylate, ethyl salicylate, dimethyl maleate,methyl benzoate, ethyl benzoate and esters of the general formulaR¹⁰C(O)O(CH₂CH₂O)_(n)R¹¹ wherein R¹⁰ is methyl or ethyl, R¹¹ is selectedfrom the group consisting of ethyl, n-propyl, isopropyl, n-butyl,isobutyl and t-butyl and n is 1 or
 2. 22. A process as claimed in claim1 wherein the water-loaded absorbing liquid which is heated in theregeneration step has a water loading of at least 10% by weight.
 23. Aprocess as claimed in claim 1 wherein the absorbing liquid-rich phaseformed in the regeneration step contains less than 1% weight of water.24. A process as claimed in claim 1 wherein the water-rich phase formedin the regeneration step contains less than 1% weight of absorbingliquid.
 25. A process as claimed in claim 24 wherein the water-richphase is contacted with a wash solvent which is immiscible with watersuch that at least part of the absorbing liquid present in thewater-rich phase is extracted into the wash solvent to produce anextract phase and a purified water phase and the extract phase is thenseparated from the purified water phase.
 26. A process as claimed inclaim 25 wherein the wash solvent has a boiling point which issubstantially lower than that of the absorbing liquid and the washsolvent is separated from the absorbing liquid contained in the extractphase as an overhead fraction by distillation.
 27. A process as claimedin claims 26 wherein the wash solvent has a boiling point which is atleast 20° C. below the boiling point of the absorbing liquid.
 28. Aprocess as claimed in claim 25 wherein the wash solvent is a light endhydrocarbon having from 5 to 10 carbons and mixtures thereof or anaromatic hydrocarbon such as toluene and xylene.